Devices generate low-energy electron beams which emerge from miniaturized vacuum chambers. They are used for electron beam hardening of rigid and flexible materials as well as in the printing sector. These sectors make different demands in terms of the electron energy and electron beam power. Relatively high electron energy levels of up to 280 keV are needed for electron beam hardening of coatings on rigid substrates, mainly on furniture parts, doors, laminated panels and strips. In these applications, sometimes layers with surface densities of 200 g/cm.sup.2 have to be hardened. Since the production speeds in the furniture sector are not determined by the hardening but rather by other process-technical steps such as positioning, grinding, etc., low to medium electron beam power levels are needed, depending on the hardness dosage.
On the other hand, if a continuous surface is to betreated such as, for example, paper or film-coated substrates, and if the layer thicknesses are below 40 g/m.sup.2, then it is also possible to work advantageously with lower electron energy levels. In the case of electron beam hardening of coatings on flexible materials, the work is often carried out from roll to roll. Here production speeds between 100 and 300 m/min are common. The surface densities of these coatings lie in the range from 1 to 30 g/m.sup.2. This calls for low electron energy levels and medium electron beam power levels.
In the printing sector, especially in roller-offset printing, the machine speeds reach 600 to 1000 m/min. The printing speeds are somewhat lower. In spite of relatively low hardness dosages of printing inks, extremely high electron beam power levels with low electron energy levels are needed.
For this purpose, devices with electron energy levels between 150 and 250 keV and beam currents between 30 and 300 mA are used. In `Nuclear Instruments & Methods in Physics Research`, Section B 1992, Article "LEA electron accelerators for radiation processing", a new device with a wire-shaped linear cathode is described in which the electrons are formed without a control grid over a tubular electrode and transferred out through an electron beam window.
Energy losses occur when the electrons are transferred out of the device, which is under a vacuum, through a radiation-permeable window made, for example, of thin titanium film. The energy loss in a 15 .mu.m titanium film amounts to about 25% for electrons with energy levels of 150 keV. Additional losses occur at the ribs of the support grid and due to the slanted incidence of the electrons striking the exit window. Electron beam windows are described in greater detail in DE 26 06 169 C2.
The lower limit of the electron energy is determined by the electron beam window. An upper limit of the electron beam power is obtained by the maximum possible current load/cm.sup.2 of window surface area, which should not exceed 0.2 mA/cm.sup.2.
In the case of such devices without control grids, it would be desirable if the energy loss during the transfer out of the electron beam window could be reduced and if equal or greater electron beam power levels could be achieved with lower electron energy levels.